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Ann Thorac Surg 1999;68:919-924
© 1999 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Aortic valve replacement: is the stentless xenograft an alternative to the homograft? midterm results

^a Department of Surgery I and Cardiology, General Hospital Linz, Linz, Austria
^b Department of Statistics, Johannes Kepler-Universität, Linz, Austria

Address reprint requests to Dr Gross, Department of Surgery I, General Hospital Linz, Krankenhausstr 9, 4020 Linz, Austria

	Abstract

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Background. This study was performed to assess the midterm clinical results after aortic valve replacement (AVR) with stentless xenograft (SX) compared with cryopreserved aortic or pulmonary homografts (HX).

Methods. In 139 patients (< 60 years) undergoing elective AVR, 59 HX and 80 SX were inserted. All patients were followed clinically and by color flow Doppler echocardiography for 45 ± 12 months (range 31–58 months).

Results. There were 5 in-hospital deaths (3.5%): 4 HX and 1 SX (p = NS). The mean gradient was 6 ± 2 mm Hg in HX versus 13 ± 6 mm Hg in SX (p < 0.001) and remained unchanged during follow-up. Actuarial survival (HX 77%, SX 80%), freedom from endocarditis (HX 91%, SX 99%), freedom from thromboembolic events (HX 98%, SX 90%), and freedom from reoperation (HX 98%, SX 100%) were comparable between groups after 58 months.

Conclusions. Despite slightly higher transvalvular gradients, the stentless aortic valve achieved excellent midterm results, when compared with homografts.

	Introduction

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Calcification, perforation, abrasion of the leaflets along the attachments of the commisural posts, and paravalvular leakage are major disadvantages of a stented biological valve. Furthermore, a rigid stent is obstructive and carries the hazards of infection, hemolysis, and embolization [1, 2]. Therefore, it is assumed that the best stent for a biological valve is the entire aortic root. The excellent results of unstented homografts support this fact by a significantly better hemodynamic performance and long-term durability compared with stented homograft valves [3, 4]. The use of homografts was initiated by Donald Ross in 1962 and still represents the gold standard in replacement of the aortic valve with bioprostheses. However, limited availability of donor organs severely restricts the widespread use of allograft valves. Stentless aortic xenograft valves were introduced in clinical practice to overcome the problems of a stented valve and to mirror the hemodynamic performance of homografts. Encouraging results for different stentless xenograft valves in respect to perioperative and midterm results have been published recently [5–8]. The aim of our randomized prospective study was to evaluate whether stentless xenografts can be true alternatives to homografts.

	Material and methods

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Xenograft
We used a stentless bioprosthesis xenograft (model 2500 Prima; Baxter Health Care Corporation, Edwards CVS Division, Irvine, CA). It is a low-pressure glutaraldehyde-treated porcine valve with the septal muscle shelf removed and a reinforced Dacron-covered inflow annulus. The resected coronary ostia are also covered with small pieces of Dacron. Sizes from 19 to 27 mm are available and were implanted.

Homograft
All cryopreserved homograft valves were procured by our own homograft bank under sterile conditions from brain-dead organ donors or cadaver donors within 24 h of death. Details of our valve processing protocol have been published previously [9]. ABO group compatibility was not regarded as mandatory.

Surgical technique
The majority of all valves (88.5%) were inserted freehand in subcoronary position, leaving the noncoronary sinus intact. The diameter of the valves had to be at least as big as the internal diameter of the patient’s aortic root, generally oversized by 1 or 2 mm. We performed the inflow suture line either with the use of six running 4-0 polypropylene sutures or 28 to 32 interrupted sutures of the same material. The outflow anastomosis was completed as a continuous suture line using two 4-0 polypropylene threads.

Eleven and one-half percent of the valves were inserted as a root replacement: 13 homografts (22.0%) were implanted as a "free root" [10], and three stentless valves (3.8%) were inserted as a "mini root" [11].

All operations were performed under standard extracorporeal circulation with moderate hypothermia. Pharmacological cardiac arrest was obtained by the use of St. Thomas cardioplegic solution. After aortic cross-clamping, an oblique-shaped aortotomy was performed, the valve excised, the annulus decalcified, and the aortic root diameter measured with commercially available sizers.

Follow-up and data analysis
Postoperatively, valve performance was assessed by color flow Doppler echocardiography. All hospital survivors completed the follow-up protocol. Patients were scheduled for follow-up examination at 6 and 12 months, and on a yearly basis thereafter. Doppler echocardiography and clinical examination were done by our cardiology staff or by the referring hospital.

Echocardiographic classification of aortic insufficiency was used in accordance with criteria published by Perry and associates [12]. No patient received anticoagulation. Between January and October 1997, all survivors had their final examination. No patient was lost to follow-up, with a mean study period of 45 ± 12 months (range 31–58 months). Infectious, thromboembolic, and bleeding complications were recorded according to the published guidelines for mortality and morbidity after cardiac valvular operations [13].

Patients
After obtaining approval of the Ethical Board of our hospital, all patients over 60 years of age were randomized preoperatively, to receive either a cryopreserved aortic or pulmonary homograft or a stentless porcine xenograft. Only patients undergoing elective aortic valve replacement with or without concomitant coronary bypass grafting were included in this study. Excluded were multiple valve replacement, urgent procedures, endocarditis, or recurrent valve replacement. Results of randomization were probably biased by three factors. (1) Treatment assignment was planned for a patient cohort of 100 patients in each group. Randomization was stopped early in February 1994 due to the now severely limited number of available homografts and the disappointing results with pulmonary homografts in aortic position. (2) The use of aortic or pulmonary homografts was dictated by availability in the assumption that both valves will perform equally. (3) No additional risk stratification for comorbidity or severity of disease was performed. This explains the disparity in patient numbers as well as small differences in pre- and intraoperative variables.

Between November 1992 and February 1994, 153 patients fulfilled the inclusion criteria. In 14 patients (9.1%) presenting with a severely calcified aorta and coronary ostia intraoperatively, a stented bioprosthesis was implanted due to expected difficulties in performing the distal anastomosis.

In the remaining 139 patients, 59 (42.4%) received a homograft (29 aortic valves, 30 pulmonary valves) and 80 (57.6%) received a stentless xenograft. Preoperative and intraoperative patient data are shown in Table 1.

Statistical methods
Continuous variables are expressed as mean ± 1 standard deviation. For the evaluation of statistically significant results in the preoperative and intraoperative data (Table 1), we used the ² test and Fisher’s exact test for nominal variables, the Mann-Whitney U-test for ordinal variables and for metric variables with marked deviations from normality, and the two-sample t test for approximately normal variables.

Eventual changes in ordinal and metric clinical variables occurring between operation and follow-ups were statistically assessed by the Wilcoxon-test or the two-sample t test for paired samples, respectively. Survival analysis for specific events of interest was performed and graphically represented by the actuarial method. Comparison of groups was based on the Wilcoxon-Gehan statistic. In general, probability values less than 0.05 were considered significant.

	Results

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Hospital mortality (30 days)
Five patients (3.5%) died during hospital stay. None of these events was valve related, as proven by postmortem section. Hospital mortality did not differ significantly between groups, with 4 patients (2.8%) in the homograft group and 1 patient (0.7%) in the xenograft group. Two cases of myocardial infarction in patients with severe coronary artery disease, one cerebral stroke intraoperatively, one sudden death on fifth postoperative day, and one mediastinitis, followed by multi-organ failure in a diabetic patient, were responsible for these fatal outcomes.

Immediate postoperative results
Four patients developed low output failure with a cardiac index less than 1.5 L/min, requiring high doses of inotropic drugs; 2 of them needed an intraaortic counterpulsation. Total heartblock in 8 patients led to implantation of a permanent pacemaker; 3 patients with preoperative renal insufficiency were on transient hemodialysis postoperatively.

One hundred thirty-six patients were examined by Doppler echocardiography before discharge. Table 2 demonstrates the mean gradients according to valve sizes and the grade of valve insufficiency.

View larger version (15K): [in this window] [in a new window]   Fig 1. Actuarial estimate. Freedom from valve-related death. HX = patient with homograft valves; SX = patient with xenograft valves. Patients at risk are reported in parentheses.

View larger version (15K): [in this window] [in a new window]   Fig 2. Actuarial estimate. Patient survival. HX = patient with homograft valves; SX = patient with xenograft valves. Patients at risk are reported in parentheses.

Two other patients with pulmonary homografts demonstrate a valvular insufficiency gradient III, 56 and 51 months postoperatively. Both patients are in good clinical condition and scheduled for reoperation.

View larger version (16K): [in this window] [in a new window]   Fig 3. Actuarial estimate. Freedom from endocarditis. HX = patient with homograft valves; SX = patients with stentless xenograft valves. Patients at risk are reported in parentheses.

View this table: [in this window] [in a new window]   Table 4. Echocardiographic Results in Patients With a Follow-up 48 Months

Thromboembolic events
Thromboembolic events (stroke, prolonged reversible ischemic neurologic deficit [PRIND], transitory ischemic attack [TIA]) were reported in 6 patients (4.4%), 5 of them in the xenograft group: 3 patients were in atrial flutter, 2 of them died. In 1 patient, the event could be ascribed to bilateral carotid stenoses; another one had this stroke during a complicated course of pacemaker implantation in another hospital. One patient with an aortic homograft suffered from a stroke 42 months postoperatively.

Freedom from actuarial thromboembolism is 98% for homografts and 90% for xenograft, respectively (p = NS).

Prosthetic valve endocarditis
Five patients (3.7%) acquired infective endocarditis in their prosthetic valve, leading to reoperation in 1 case and to death in 3 cases. One patient could be cured conservatively.

In 1 patient with an aortic homograft, endocarditis occurred 6 weeks postoperatively and was caused by Candida albicans. One patient with an aortic homograft died 42 months postoperatively after pacemaker infection.

Septicemia and toxic shock were the cause of death in another patient with pulmonary homograft 24 months postoperatively. One patient with a pulmonary homograft had infective endocarditis 36 months postoperatively and had to be reoperated.

Endocarditis in a patient with xenograft could be cured conservatively; this patient died 1 year later of a carcinoma of the colon. No risk factor could be detected in these 3 patients.

Diagnosis of endocarditis was based on positive blood cultures or histologic confirmation at reoperation or autopsy. Freedom from actuarial endocarditis is 91% for homografts and 99% for xenografts, respectively (p = NS) (Fig 3).

	Comment

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The search for an ideal valve substitute for aortic valve replacement (AVR) is continual and persistent. Homografts as well as stentless bioprostheses restore the complex anatomy of the aortic root and allow more physiological excursions of the valvular cusps with the consequence of reduction of mechanical stress, which is suspected to be a major factor for durability and calcification [14, 15]. The lack of a rigid stent diminishes the danger of wear stress, paravalvular leakage, thromboembolism, and hemolysis.

The homograft, introduced by Donald Ross, is the "gold standard" in AVR with stentless valves for more than 30 years, and excellent results have been achieved. On the other hand, restricted availability of donor organs and sizing issues remain a limiting factor. Stentless aortic xenografts were introduced to copy the homograft performance and to add immediate availability of all sizes. Excellent short- and midterm results have been published for different xenograft types in previous years [6, 7, 16, 17]. Therefore, it is reasonable to assume that the stentless xenograft could be a true alternative to homografts for AVR.

In our series, xenograft valves achieved excellent hemodynamic results compared with homografts. There is a slight, but significantly higher, transprosthetic gradient at rest observed in the xenograft group. Multiple factors account for this difference. The cusps of the stentless valves are glutaraldehyde preserved and thus more likely to be less flexible. Additionally, the Dacron-reinforced sewing ring is more rigid than the smooth and flexible homograft. Furthermore, the inward folding of the Dacron cloth beneath the coronary ostia is also considered to be an obstructing factor [18, 19]. A new, currently prepared valve design should overcome these limitations. The higher gradient remains persistent over the follow-up period with only partial regression. Whether this difference in peak and mean gradients is of any consequence may be doubtful. The results in our series concerning late complications, death, and clinical outcome after 5 years of follow-up are comparable. The ideal technique of implantation of a stentless valve either as subcoronary implantation or root replacement, or as "mini-root," is still under discussion [20]. The subcoronary implantation technique, used in the majority of our cases, seems to be a safe and reproducible method. Only two cases of minor insufficiency in the homograft group immediately postoperatively confirm this. Proper sizing and meticulous implantation technique requires some experience. In distorted roots, very small roots, or in patients with significant dilatation of the sinotubular junction, the freestanding root replacement technique seems to be the appropriate method for AVR, utilizing these prostheses.

The longer neck of the homograft cylinders makes this technique easier than with the stentless xenograft. The intraaortic cylinder technique or "mini-root" implies many disadvantages, such as difficult sizing, distortion of the coronary arteries, and inappropriate wrapping of the aortic wall. Our experience with this kind of technique is limited. There are 3 cases of incompetence during the follow-up period, all of them in pulmonary homograft patients. One patient had to be reoperated; the other 2 patients refused reoperation at that time.

These data confirm our experience with pulmonary homografts in a larger series [21], which had a high incidence of intrinsic graft failure and was therefore abandoned for AVR.

Implantation of a stentless valve is technically more demanding and needs longer ischemic times compared with traditional stented valves, but this did not influence postoperative outcome. The firmness of the tissue and the Dacron reinforcement of the suture lines make the implantation of a stentless xenograft easier, when compared with the pliable homograft.

The rate of infective endocarditis is surprisingly high, especially in the homograft group (4 vs 1, respectively, p = NS). In 1 case a probable fungal contamination of the homograft was responsible for the fatal outcome. In the remaining 3 cases in the homograft group, 2 belonged to patients with pulmonary homografts, a trend that has been reported in a larger series from our institution [21]. The high incidence of endocarditis in this series was another argument for us to abandon the pulmonary homograft. The only case in the xenograft group could be cured conservatively without any sequelae.

The rate of thromboembolic events is not unusual, considering the age of the patients. Three of the 5 cases suffered from arrhythmic problems; in 1 patient this event can be ascribed to atherosclerosis of both carotid arteries. The 2 cases of reoperation belonged to the homograft group: 1 patient with endocarditis and the other with intrinsic graft failure; both patients had to be reoperated and recovered fully.

Our analysis demonstrates that stentless xenograft seems to be a suitable alternative to the homograft for AVR in patients older than 60 years. Whether long-term durability and valve performance continue to be comparable requires further studies. There is a slight, but statistically significant, higher gradient in the xenograft group, when compared with homografts. Endocarditis, thromboembolic events, and late deaths are comparable in both groups. The pulmonary homograft implies many disadvantages and should not be used any more. The implantation technique for stentless xenograft is even easier than for homografts and less time consuming. The physiologic concept of stentless valves and the excellent midterm performance makes us believe that these valves might perform equally to aortic homografts in long-term follow-up.

	References

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